Refinement of the hydrodynamical model for the radiative transfer simulation via the extrapolation and interpolation of the dust densities for a single SPH particle. In the left panel, starting from dust density values obtained with SPH simulations (red dots), we perform a linear fit for log ρ_d values associated with the two smallest simulated grain sizes to obtain densities for grains down to 5 nm (blue line). Then, we use a cubic spline interpolation on the remaining log ρ_d values (orange line). In the right panel, with the linear fit, we obtain ten dust densities related to grain sizes between 5 nm and 0.37 µm (to the left of the vertical dashed gray line), while with the cubic spline interpolation we obtain 50 densities for grain sizes between 0.37 µm and 1 mm (to the right of the vertical dashed gray line).